This invention relates to a spin transistor.
Traditional silicon transistors, as is well known, operate through the movement of electrons and holes under the effect of an electric field. Until recently, the fact that some of the electrical carriers have a spin-up configuration and some a spin-down configuration, was ignored.
M. Johnson, in Science, 260, 320 (Apr. 16, 1993), described the first all metal transistor, known as a spin transistor or magnetic transistor. This device comprises a paramagnetic layer sandwiched between two outer ferromagnetic layers to make a trilayer structure. By connecting a terminal to each of the layers, a three-terminal giant magnetoresistive (GMR) device may be formed; by analogy with a traditional silicon transistor, the two outer ferromagnetic layers are referred to as the collector and emitter, and the paramagnetic layer is referred to as the base.
The device operates by pumping electrical current from the ferromagnetic emitter to the paramagnetic base, thus creating a spin accumulation (an excess of up-spin carriers over down-spin carriers or vice versa) in the paramagnetic material. The base can thus be thought of as suffering a divergence in the chemical potentials of the two spin channels (up-spin and down-spin). The collector current is then magnetically dependent.
The Johnson device has two major drawbacks. Firstly, in practice the transistor offers no power gain. Secondly, the signal amplitudes involved are very low (of the order of nanovolts).
More recent developments in the spin transistor have employed two Schottky diodes (silicon/metal stack/silicon), as described by D. Monsma et al. in Physics Review Letters 74, 5260; 1995.
This hybrid device exploits the rectifying properties of semiconductor junctions. The silicon outer layers act as emitter and collector respectively, whilst the intermediate layer (in fact a GMR multilayer) acts as the base. The transistor is biassed to pass current from the (silicon) emitter to the GMR base and the magnetic configuration of the latter governs the ultimate collector current. In particular, the magnetic configuration of the base determines the scattering length within it, which in turn determines the number of charge carriers able to clear the base/collector Schottky barrier. Thus, the device acts as a three-terminal device analogous to a bipolar junction transistor but with the ratio of collector-to-emitter current, i.e. the current gain, being a function of externally applied magnetic field.
The Monsma device, whilst being an excellent magnetic field sensor, still suffers in comparison with traditional silicon transistors. Whilst the ratio of the collector current to base current (β) varies by a large amount, the absolute value of β is less than unity (typically eight orders of magnitude less than a commercial bipolar junction transistor). The Monsma spin transistor has also proved difficult to manufacture commercially because of the need to use cold welding techniques to assemble the device.
WO97/41606 shows a device representing a further improvement to the above. Here, a spin transistor is described which exploits the behaviour of spin-polarized currents in the silicon itself. The spin transistor of WO97/41606 is essentially a three-terminal device with a silicon base, emitter and collector. The emitter includes a spin injector to inject a spin-polarized diffusion current into the silicon base, with a barrier layer employed between the base and a silicon collector. The barrier layer is responsive to a magnetic field and acts to alter the base-collector current.
The device of WO97/41606 thus provides the benefits of a traditional transistor (large β), because it is partly a silicon device and the carrier transport is thus diffusion-driven and not field-driven in the base. Nonetheless, the device has a degree of magnetic sensitivity because of the use of the barrier layer in combination with spin-polarized carriers. However, the device does still suffer a number of drawbacks. Firstly, the spin-polarized carriers are injected directly into a region where they are majority carriers, and this causes dilution of the spin such that a significant reduction in the number of spin-polarized carriers reaches the collector. Secondly, injecting from a magnetic region directly into silicon across an Ohmic or Schottky barrier causes a strong depolarizing effect, and it is believed that interface contamination or interdiffusion at the Ohmic contact may be one cause of this.
It is an object of the present invention to provide an improved spin transistor which at least alleviates these problems with the prior art.